1. Field of the Invention
The present invention relates to a drilling method, and to a drilling apparatus. Embodiments of the invention relate to a drilling method and apparatus where the effective circulating density (ECD) of drilling fluid (or drilling xe2x80x9cmudxe2x80x9d) in communication with a hydrocarbon-bearing formation is lower than would be the case in a conventional drilling operation. The invention also relates to an apparatus for reducing the buildup of drill cuttings or other solids in a borehole during a drilling operation; and to a method of performing underbalance drilling.
2. Description of the Related Art
When drilling boreholes for hydrocarbon extraction, it is common practice to circulate drilling fluid or xe2x80x9cmudxe2x80x9d downhole: drilling mud is pumped from surface down a tubular drillstring to the drill bit, where the mud leaves the drillstring through jetting ports and returns to surface via the annulus between the drillstring and the bore wall. The mud lubricates and cools the drill bit, supports the walls of the unlined bore, and carries dislodged rock particles or drill cuttings away from the drill bit and to the surface.
In recent years the deviation, depth and length of wells has increased, and during drilling the mud may be circulated through a bore several kilometres long. Pressure losses are induced in the mud as it flows through the drillstring, downhole motors, jetting ports, and then passes back to the surface through the annulus and around stabilisers, centralisers and the like. This adds to natural friction associated pressure loss as experienced by any flowing fluid.
Similarly, the pressure of the drilling mud at the drill bit and, most importantly, around the hydrocarbon-bearing formation, has tended to rise as well depth, length and deviation increase; during circulation, the pressure across the formation is the sum of the hydrostatic pressure relating to the height and density of the column of mud above the formation, and the additional pressure required to overcome the flow resistance experienced as the mud returns to surface through the annulus. Of course the mud pressure at the bit must also be sufficient to ensure that the mud flowrate through the annulus maintains the entrainment of the drill cuttings.
The mud pressure in a bore is often expressed in terms of the effective circulating density (ECD), which is represented as the ratio between the weight or pressure of mud and the weight of a corresponding column of water. Thus, the hydrostatic pressure or ECD at a drill bit may be around I.05SG,; whereas during circulation the mud pressure, or ECD, may be as high as I.55SG.
It is now the case that the ECD of the drilling mud at the lower end of the bore where the bore intersects the hydrocarbon-bearing formations is placing a limit on the length and depth of bores which may be drilled and reservoirs accessed. In addition to mechanical considerations, such as top drive torque ratings and drill pipe strength, the increase in ECD at the formation may reach a level where the mud damages the formation, and in particular reduces the productivity of the formation. During drilling it is usually preferred that the mud pressure is higher than the fluid pressure in the hydrocarbon-bearing formation, such that the formation fluid does not flow into the bore. However, if the pressure differential exceeds a certain level, known as the fracture gradient, the mud will fracture the formation and begin to flow into the formation. In addition to loss of drilling fluid, fracturing also affects the production capabilities of a formation. Attempts have been made to minimise the effects of fracturing by injecting materials and compounds into bore to plug the pores in the formation. However, this increases drilling costs, is often of limited effectiveness, and tends to reduce the production capabilities of the formation.
High mud pressure also has a number of undesirable effects on drilling efficiency. In deviated bores the drillstring may lie in contact with the bore wall, and if the bore intersects a lower pressure formation the fluid pressure acting on the remainder of the string will tend to push the string against the bore wall, significantly increasing drag on the string; this may result in what is known as xe2x80x9cdifferential stickingxe2x80x9d.
It has also been suggested that high mud pressure at the bit reduces drilling efficiency, and this problem has been addressed in U.S. Pat. No. 4,049,066 (Richey) and U.S. Pat. No. 4,744,426 (Reed), the disclosures of which are incorporated herein by reference. Both documents disclose the provision of pump or fan arrangements in the annulus rearwardly of the bit, driven by mud passing through the drillstring, which reduces mud pressure at the bit. It is suggested that the disclosed arrangements improve jetting and the uplift of cuttings.
Another method of reducing the mud pressure at the bit is to improve drillstring design to minimise pressure losses in the annulus, and U.S. Pat. No. 4,823,891 (Hommani et al) discloses a stabiliser configuration which aims to minimise annulus pressure losses, and thus allow a desired mud flow to be achieved with lower initial mud pressure.
It is also known to aerate drilling mud, for example by addition of nitrogen gas, however the apparatus by necessary to implement this procedure is relatively expensive, cuttings suspension is poor, and the circulation of two phase fluids is problematic. The presence of low density gas in the mud may also make it difficult to xe2x80x9ckillxe2x80x9d a well in the event of an uncontrolled influx of hydrocarbon fluids into the wellbore.
It is among the objects of embodiments of the present invention to obviate or alleviate these and other difficulties associated with drilling operations.
According to the present invention there is provided a drilling method in which a drill bit is mounted on a tubular drill string extending through a bore, the method comprising:
drilling a bore which extends through a formation containing fluid at a predetermined pressure;
circulating drilling fluid down through the drill string to exit the string at or adjacent the bit, and then upwards through an annulus between the string and bore wall; and
adding energy to the drilling fluid in the annulus at a location above said formation such that the pressure of the drilling fluid above said location is higher than the pressure of the drilling fluid below said location and there is a predetermined differential between the pressure of the formation fluid and the pressure of the drilling fluid in communication with the formation.
The invention also relates to apparatus for use in implementing this method.
The method of the present invention allows the pressure of the drilling fluid in communication with the formation, typically a hydrocarbon-bearing formation, to be maintained at a relatively low level, even in relatively deep or highly deviated bores, while the pressure in the drilling fluid above the formation may be maintained at a higher level to facilitate drilling fluid circulation and cuttings entrainment.
The differential between the drilling fluid pressure and the formation fluid pressure, which is likely to have been determined by earlier surveys, may be selected such that the drilling fluid pressure is high enough to prevent the formation fluid from flowing into the bore, but is not so high as to fracture or otherwise damage the formation. In certain embodiments, the pressure differential may be varied during a drilling operation to accommodate different conditions, for example the initial pressure differential may be controlled to assist in formation of a suitable filter cake. Alternatively, the drilling fluid pressure may be selected to be lower than the formation fluid pressure, that is the invention may be utilised to carry out xe2x80x9cunderbalancexe2x80x9d drilling; in this case the returning drilling fluid may carry formation fluid, which may be separated from the drilling fluid at surface.
Preferably, energy is added to the drilling fluid by at least one pump or fan arrangement. Most preferably, the pump is driven by the fluid flowing down through the drillstring, such as in the arrangements disclosed in U.S. Pat. Nos. 4,049,066 and 4,744,426. Fluid driven downhole pumps are also produced by Weir Pumps Limited of Cathcart, Glasgow, United Kingdom. The preferred pump form utilises a turbine drive, that is the fluid is directed through nozzles onto turbine blades which are rotated to drive a suitable impeller acting on the fluid in the annulus. Such a turbine drive is available, under the TurboMac trade mark, from Rotech of Aberdeen, United Kingdom. When using the preferred pump form the initial pump pressure at surface will be relatively high, as energy is taken from the fluid, as it flows down through the string, to drive the pump. Alternatively, in other embodiments it may be possible for the pump to be driven by a downhole motor, to be electrically powered, or indeed driven by any suitable means, such as from the rotation of the drillstring.
Energy may be added to the drilling fluid in the annulus at a location adjacent the drill bit, but is more likely to be added at a location spaced from the drill bit, to allow the bore to be drilled through the formation and still ensure that the higher pressure fluid above said location is spaced from the formation.
In one embodiment of the invention, a proportion of the circulating drilling fluid may be permitted to flow directly from the drillstring bore to the annulus above the formation, and such diversion of flow may be particularly useful in boreholes of varying diameter, the changes in diameter typically being step increases in bore diameter. When the bore diameter increases, drilling fluid flow speed in the annulus will normally decrease, and the additional volume of fluid flowing directly from the drillstring bore into the annulus assists in maintaining flow speed and cuttings entrainment. This may be achieved by provision of one or more bypass subs in the string. The bypass subs may be selectively operable to provide fluid bypass only when considered necessary or desirable.
The drill string may also incorporate means for isolating sections of one or both of the drill string bore and annulus when there is no fluid circulation. This is of particular importance when the pressure of the circulating drilling fluid at the formation is lower than hydrostatic pressure; the isolating means will support the column of fluid above the formation, allowing lower sections of the bore to be maintained at relatively low pressures. Alternatively, or in addition, the isolating means may serve to prevent fluid flowing from the formation and then up the bore in underbalance conditions. The isolating means may be in the form of one or more valves, packers, swab cups or the like.
The drillstring may also be provided with means for agitating cuttings in the annulus, such as the flails disclosed in U.S. Pat. No. 5,651,420 (Tibbets et al), the disclosure of which is incorporated herein by reference. Tibbets et al propose mounting flails on elements of the drillstring, which flails are actuated; by the rotation of the string or the flow of drilling fluid around the flails. Most preferably however, the agitating means are mounted on a body which is rotatable relative to the string. The body is preferably driven to rotate by drive means actuated by the flow of drilling fluid through the string, but may be driven by other means. This feature may be provided in combination with or separately of the main aspect of the invention.